An image forming device such as an electrographic printer is driven with AC power supply (AC voltage), and a power supply control for a heater to heat a fusing roller is made by using a switching element such as a triode ac (TRIAC).
A change in resistance of the heater for heating the fusing roller as a function of temperature is wide, and an inrush current corresponding to a current that is several times that of a rated current may instantly flow into the heater when the heater is cold and the internal resistance is small. When the inrush current that is overcurrent flows into the heater, the heater may be overloaded and damaged.
Therefore, it is proposed that the image forming device have a zero-crossing detecting circuit that detects the zero-crossing point of AC voltage and outputs a zero-crossing signal each time the zero-crossing point is detected in order to control the switching element to turn ON when the AC voltage passes the point of zero charge (zero-crossing point) to reduce the inrush current to the heater. As the zero-crossing detecting circuit, for example as disclosed in Japanese laid-open application 2006-258698, it is proposed that the zero-crossing detecting circuit includes a first photocoupler, which detects a positive half-wave of AC voltage and outputs a square-wave corresponding to the half-wave, and a second photocoupler, which detects a negative half-wave of AC voltage and outputs a square-wave corresponding to the half- The proposed circuit outputs the zero-crossing signal by detecting the zero-crossing point based on the square-waves output from each photocoupler.
FIG. 5 illustrates a related zero-crossing detecting circuit 300, and FIGS. 6A-6F illustrate signal waveforms that respective parts of the related zero-crossing detecting circuit 300 output. In the related zero-crossing detecting circuit 300, when a current I1, which is represented by the waveform shown in FIG. 6B, flows into a photodiode (light-emitting element) in a photocoupler 301 based on the positive half-wave of an AC voltage E shown in FIG. 6A, the photodiode emits light. The emitted light is received by a phototransistor (light-receiving element) in the photocoupler 301.
Since the phototransistor in the photocoupler 301 turns ON when it receives the light from the photodiode mentioned above, it outputs an output signal F1, which is a square-wave shown in FIG. 6D, to a signal converter 303.
Similarly, in the related zero-crossing detecting circuit 300, when a current I2, which is represented by the waveform shown in FIG. 6C, flows into a photodiode in a photocoupler 302 based on a negative half-wave of the AC voltage E shown in FIG. 6A, the photodiode emits light. The emitted light is received by a phototransistor in the photocoupler 302.
The phototransistor in the photocoupler 302 turns ON by receiving the light mentioned above, therefore an output signal F2, which is a square-wave as shown in FIG. 6E, is output to the signal converter 303.
When the output signal F1, which is represented by the square-wave shown in FIG. 6D, and the output signal F2, which is represented by the square-wave shown in FIG. 6E, are input, the signal converter 303 converts the output signals Fl and F2 respectively based on OR (logical sum), and outputs a zero-crossing signal P0 that is a pulse signal showing a rising timing of each signal. The waveform diagram of the zero-crossing signal P0 is illustrated in FIG. 6F.
However, since the related zero-crossing detecting circuit 300 mentioned above is configured to send current constantly to the photocouplers 301 and 302 so as to output the zero-crossing signal P0, there is a problem that the zero-crossing detecting device or the image forming device in which it is installed consumes a relatively large amount of power.
In view of this problem, the invention provides a zero-crossing detecting device and an image forming device including the zero-crossing detecting device that are driven with low power consumption and are able to detect a zero-crossing point of AC voltage.